1. Field of the Invention
The present invention relates to an optical disc reproducing apparatus, and more particularly, to an optical disc reproducing apparatus for reproducing an eight to fourteen modulation ((EFM)/EFM+) signal using a viterbi detector.
2. Description of the Related Art
As multimedia technology, including audio and video technologies, and related data compression technology, continue to rapidly develop, recording media for supporting these technologies continues to evolve. Key requirements for new recording media include an ability to transmit high-speed data and a large storage capacity. As the successor to the magnetic tape, the popular compact disc and video compact disc are capable of storing approximately 600 Mbytes of audio and video data in an optical disc format of a diameter of 12 cm. Recently, digital video discs, a digital video disc ROMs, and digital video disc RAMs have been produced that are capable of storing multimedia data or programs of 4.7 Gbytes in an optical disc of 12 cm diameter. For the compact disc, data is stored following modulation of 8-bit data to 14-bit data using the popular EFM method. For the digital video disc, data is stored after modulating the 8-bit data to 16-bit data using the popular EFM+method.
The system for reproducing data stored in an optical disc is commonly referred to in the art, and herein, as an optical disc reproducing apparatus.
FIG. 1 shows a portion of a conventional optical disc reproducing apparatus. Referring to FIG. 1, the conventional optical disc reproducing apparatus includes a current-to-voltage converter 111, an automatic gain controller 121, an analog equalizer 131, and a slice circuit 141. Combined, the components reproduce an EFM/EFM+ signal. A high frequency signal stored on an optical disc (not shown), is read by an optical pick-up (not shown), and is converted into a high frequency signal with respect to a voltage by the current-to-voltage converter 111. Any distorted portions of the signal output from the current-to-voltage converter 111 are compensated for by an optical characteristic in the automatic gain controller 121. The signal output by the automatic gain controller 121 is input to the slice circuit 141 through an analog equalizer 131 and the resulting EFM/EFM+ signal is generated by the slice circuit 141.
In the conventional optical disc reproducing apparatus described above, various errors are generated during reproduction of the EFM/EFM+ signal by interference between signals when the manufacturing state of a disc is poor or unreliable, when a disc is recorded in high density, or when the operation speed of the optical disc reproducing apparatus increases.
According to U.S. Pat. No. 5,684,773, a viterbi decoder is employed for a digital data information reproducing apparatus. However, since the difference metric of a signal SP(k) input to a branch-metric operator is obtained by the branch-metric operator included in the viterbi detector using five variables y(ak), the calculation method becomes complicated in branch-metric operating. Accordingly, the structure of the circuit of the viterbi detector becomes complicated and cumbersome.
To address the aforementioned limitations, it is an object of the present invention to provide a viterbi detector that reproduces a stable eight to fourteen modulation (EFM)/EFM+ signal.
It is another object of the present invention to provide an optical disc reproducing apparatus including such a viterbi detector.
To achieve the first object, there is provided a viterbi detector, comprising a branch metric calculator for receiving a signal constituted of n bits, comparing the signal with previously set three variables, and calculating a branch metric with respect to the input signal, a path metric update and storage unit for receiving the branch metric, adding the branch metric to values stored therein, comparing all the possible paths with each other, and storing a metric with respect to the path having the smallest amount of errors, and a path memory for receiving the output signal of the path metric update and storage unit and generating a viterbi decoded recovery signal.
The branch metric calculator preferably comprises a difference metric calculator for comparing the input signal with the three variables, obtaining the differences between the input signal and the three variables, and for generating three difference metrics, and a branch metric selector for generating a plurality of branch metrics by comparing the difference metrics with each other.
The variables are preferably constituted of a reference value, a limit value obtained by adding a predetermined difference value to the reference value, and a low limit value obtained by subtracting the predetermined difference value from the reference value and the reference value is obtained by 2nxe2x88x921. The variables are previously set.
To achieve the second object, there is provided an optical disc reproducing. apparatus for an optical disc in which digital data is stored, comprising an optical pick-up for reading digital data stored in the optical disc and generating a high frequency signal, an analog-to-digital converter for sampling and quantizing the high frequency signal, and a viterbi detector for decoding a quantized signal output from the analog-to digital converter and reproducing an eight to fourteen modulation (EFM)/EFM+ signal, wherein the viterbi detector comprises a branch metric calculator for receiving a signal constituted of n bits, comparing the signal with previously set three variables, and calculating a branch metric with respect to the input signal, a path metric update and storage unit for receiving the branch metric, adding the branch metric to values stored therein, comparing all the possible paths with each other, and storing a metric with respect to the path having the smallest amount of errors, and a path memory for receiving the output signal of the path metric update and storage unit and generating a viterbi decoded recovery signal.
The optical disc reproducing apparatus preferably further comprises an adaptive equalizer for compensating for the distorted high frequency component of the output signal of the analog-to-digital converter and transmitting the compensated high frequency component to the viterbi detector.
The EFM/EFM+ signal is stably reproduced by the optical disc reproducing apparatus according to the present invention.